//继承父类属性
function limitDrag(id){
    Drag.call(this,id)
 }
 //继承父类方法
 for(var i in Drag.prototype){
   limitDrag.prototype[i]=Drag.prototype[i]
 }
 limitDrag.prototype.move=function(ev){
    var event=ev||window.event;
    var l=event.clientX-this.disx;
    var t=event.clientY-this.disy;
    if(l<0) l=0;
    if(l>document.documentElement.clientWidth-this.odiv1.offsetWidth)   
    l=document.documentElement.clientWidth-this.odiv1.offsetWidth;
    this.odiv1.style.left=l+'px';
    this.odiv1.style.top=t+'px';
 }
 
 